Raster output scanning is commonly used in a number of applications, including flatbed scanning recorders, capstan imagesetters and platesetters and even some external or internal drum systems. In a typical flat field imaging application, a photosensitive material, for example, is moved at constant speed by a capstan roller or other linear conveyor means, fed from continuous rolls or precut cassettes of stacked material to present the material to a stationary optical system for scanning. Alternatively, stationary photo-sensitive material may be imaged by translating the optical system. The scan optic in the flat field imaging context typically consists of a resonating or rotating single facet mirror, or an assembly of two or more mirrors, or a glass prism consisting of one or two reflecting surfaces. It is also common to have a rotating polygon or hologon having multiple reflective or refractive facets symmetric to a central rotating axis.
Within the flat field applications, a rotating polygon scan optic has greater potential for speed and efficiency compared to the use of the resonating scanner or single facet rotary scanner. The polygon scan optic is presently preferred because for each rotation thereof, a polygon scan optic having “n” number of facets produces “n” number of scan lines, whereas for the single facet rotary scan optic or resonating scanner, each revolution of the scan optic produces only one scan line. Thus, to obtain a high resolution image on a given imaging surface in as short an amount of time as possible, it is desirable to maximize the scan rate of the scan optic.
However, in flatbed raster recording systems, such as computer-to-plate systems, technical and cost hindrances limit the practical length of the scan line. This generally limits the page width of these systems to a range of about 6 to about 24 inches.
In an attempt to overcome this page width limitation, efforts have been made, with limited success, to combine the partial scan lines produced by more than one imaging source, or produced by one source at different times, into a single, composite image. For example, it is common to write two or more adjacent pages and mechanically stitch them together. However, it is difficult to precisely butt two printed pages together. As a result, the seams between the partial scan lines produced by each imaging source can be highly visible in the composite image.
Additional artifacts produced by cross-scan errors, pixel size variations, exposure variations and other factors, may also be introduced, further diminishing the quality of the composite image. An undesirable source of error in optical scanning systems results from in-scan error. In-scan error specifically refers to errors in the placement of scan lines in a direction parallel to the lines themselves. Sources of in-scan errors may include variations in the rotational speed of the polygon mirror, pixel timing, clock jitter and mechanical errors such as bearing noise and facet rotational error.
Past attempts to join multiple image segments together have been largely unsuccessful at eliminating a visible seam where the image segments have been joined or have involved complex processing. The present invention incorporates many unique features which eliminate these and other problems associated with the use of multiple image segments to create a single larger image.
In summary, the prior art has yet to disclose or teach singularly, or in any combination, and there continues to be a significant need for, a method and apparatus for combining (“stitching”) multiple image segments to form a larger composite image without a visible seam where the image segments have been joined.